Optical modulators for equalizing the power across spectral ranges are important components in optical networks employing multiple wavelengths of light (Wavelength Division Multiplexed or WDM networks). Typically in WDM networks, especially in long-haul fibers, rare earth doped fiber amplifiers are used to replenish losses. In order to minimize non-linear effects in these amplifiers and to permit the use of common communication circuitry for all channels, it is necessary that the various wavelength channels have similar power levels. However, as various wavelengths are added or subtracted at various nodes in the network, the power levels of the various channels change, in turn changing the gain spectrum of the amplifiers. Such network changes can be slow as when customers are added and removed or fast as when network traffic is dynamically rerouted for improved efficiency. Often the variation in power is roughly monotonic in wavelength and can be adequately compensated by a monotonic change in attenuation as the wavelength changes, i.e. by a spectral attenuation tilt.
U.S. Pat. No. 5,500,761 issued to K. W. Goossen et al on Mar. 19, 1996 describes a Mechanical Anti-Reflection Switch modulator (MARS modulator) useful for power equalization. The MARS modulator is basically a Fabry-Perot cavity comprising the air gap between an optical membrane and a substrate. Modulation is based on voltage-controlled vertical movement of the membrane in relation to the substrate. The MARS modulator provides broad spectrum, high contrast reflection modulation at rates in excess of several Mbit/sec. A MARS modulator having low insertion loss and broad operating bandwidth particularly advantageous for optical communications applications is described in applicant Goossen's copending U.S. patent application Ser. No. 08/901,050 filed Jul. 25, 1997 and entitled "Micromechanical Modulator Having Enhanced Performance". Both U.S. Pat. No. 5,500,761 and application Ser. No. 08/901,050 are incorporated herein by reference.
Referring to the drawings, FIG. 1 is a schematic cross section of a single unit MARS modulator of the type described in the Goossen patent and copending application. The device 9 comprises a substrate 10 and a membrane 15 spaced from each other to define a gap 20 between them. The substrate 10 is a conductive material such as doped silicon, and the membrane 15 comprises one or more layers of conductive material such as an overlayer 15a of silicon nitride and an underlayer 15b of polycrystalline silicon. The overlayer has an index of refraction approximately equal to the square root of the substrate refractive index and the underlayer has an index of refraction approximately equal to the substrate refractive index. The thicknesses of 15a and 15b are each less than one-quarter of the operating wavelength .lambda.. The membrane 15 and the substrate 10 are spaced apart by a peripheral support layer 12 of insulating material. Electrodes 30 and 31 permit connection of the membrane 15 and substrate 10, respectively, to the terminals of a bias voltage source 29.
The air gap 20 can be controlled by a bias voltage between the substrate and the membrane. Relative reflective maxima are produced when the gap 20 is an odd integer multiple of one-quarter of the operating wavelength .lambda.. Minima are produced when the gap 20 is 0 or an even integer multiple of .lambda./4.
The modulator can employ mirrors of unequal reflectivity to provide broad operating bandwidth with low insertion loss. A high reflectivity membrane provides low insertion loss while a lower reflectivity substrate maintains the broader bandwidth of a low finesse device.
While these MARS modulators can be used to achieve a desired level of attenuation at the middle wavelength of a spectral band, this attenuation is typically produced by only one spacing between the membrane and the substrate, which means that the attenuation versus wavelength characteristic (spectral tilt) is fully defined. Thus there is no independent control over the attenuation and the spectral tilt.